Dynamic path selection in a storage network

ABSTRACT

Managing input/output (‘I/O’) queues in a data storage system, including: receiving, by a host that is coupled to a plurality of storage devices via a storage network, a plurality of I/O operations to be serviced by a target storage device; determining, for each of a plurality of paths between the host and the target storage device, a data transfer maximum associated with the path; determining, for one or more of the plurality of paths, a cumulative amount of data to be transferred by I/O operations pending on the path; and selecting a target path for transmitting one or more of the plurality of I/O operations to the target storage device in dependence upon the cumulative amount of data to be transferred by I/O operations pending on the path and the data transfer maximum associated with the path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priorityfrom U.S. Pat. No. 10,289,344, issued May 14, 2019, which is acontinuation application of and claims priority from U.S. Pat. No.10,001,951, issued Jun. 19, 2018, which is a continuation application ofand claims priority from U.S. Pat. No. 9,760,297, issued Sep. 12, 2017.

BACKGROUND Technical Field

The field of the invention is data processing, or, more specifically,methods, apparatus, and products for managing input/output (′I/O′)queues in a data storage system.

Background Art

Enterprise storage systems can include many storage devices that areavailable for use by users of many types. Users of enterprise storagesystems may initiate I/O operations such as, for example, reading datafrom a storage device in the storage system, writing data to a storagedevice in the storage system, and so on. Such I/O operations musttypically pass through various software and hardware modules.

SUMMARY OF INVENTION

Methods, apparatuses, and products for managing I/O queues in a datastorage system, including: receiving, by a host that is coupled to aplurality of storage devices via a storage network, a plurality of I/Ooperations to be serviced by a target storage device; determining, foreach of a plurality of paths between the host and the target storagedevice, a data transfer maximum associated with the path, wherein thedata transfer maximum specifies a cumulative amount of data that may beassociated with I/O operations pending on the path; determining, for oneor more of the plurality of paths, a cumulative amount of data to betransferred by I/O operations pending on the path; and selecting atarget path for transmitting one or more of the plurality of I/Ooperations to the target storage device in dependence upon thecumulative amount of data to be transferred by I/O operations pending onthe path and the data transfer maximum associated with the path.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescriptions of example embodiments of the invention as illustrated inthe accompanying drawings wherein like reference numbers generallyrepresent like parts of example embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 sets forth a block diagram of a storage system configured formanaging I/O queues according to embodiments of the present disclosure.

FIG. 2 sets forth a block diagram of a storage array controller usefulin managing I/O queues in a data storage system according to embodimentsof the present disclosure.

FIG. 3 sets forth a block diagram of a storage system configured formanaging I/O queues according to embodiments of the present disclosure.

FIG. 4 sets forth a flow chart illustrating an example method formanaging I/O queues in a data storage system according to embodiments ofthe present disclosure.

FIG. 5 sets forth a flow chart illustrating an additional example methodfor managing I/O queues in a data storage system according toembodiments of the present disclosure.

FIG. 6 sets forth a flow chart illustrating an additional example methodfor managing I/O queues in a data storage system according toembodiments of the present disclosure.

FIG. 7 sets forth a flow chart illustrating an additional example methodfor managing I/O queues in a data storage system according toembodiments of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Example methods, apparatus, and products for managing I/O queues in adata storage system in accordance with the present invention aredescribed with reference to the accompanying drawings, beginning withFIG. 1. FIG. 1 sets forth a block diagram of a storage system configuredfor managing I/O queues according to embodiments of the presentdisclosure. The storage system of FIG. 1 includes a number of computingdevices (164, 166, 168, 170). Such computing devices may be implementedin a number of different ways. For example, a computing device may be aserver in a data center, a workstation, a personal computer, a notebook,or the like.

The computing devices (164, 166, 168, 170) in the example of FIG. 1 arecoupled for data communications to a number of storage arrays (102, 104)through a storage area network (SAN′) (158) as well as a local areanetwork (160) (‘LAN’). The SAN (158) may be implemented with a varietyof data communications fabrics, devices, and protocols. Example fabricsfor such a SAN (158) may include Fibre Channel, Ethernet, Infiniband,Serial Attached Small Computer System Interface (‘SAS’), and the like.Example data communications protocols for use in such a SAN (158) mayinclude Advanced Technology Attachment (‘ATA’), Fibre Channel Protocol,SCSI, iSCSI, HyperSCSI, and others. Readers of skill in the art willrecognize that a SAN is just one among many possible data communicationscouplings which may be implemented between a computing device (164, 166,168, 170) and a storage array (102, 104). For example, the storagedevices (146, 150) within the storage arrays (102, 104) may also becoupled to the computing devices (164, 166, 168, 170) as networkattached storage (‘NAS’) capable of facilitating file-level access, oreven using a SAN-NAS hybrid that offers both file-level protocols andblock-level protocols from the same system. Any other such datacommunications coupling is well within the scope of embodiments of thepresent disclosure.

The local area network (160) of FIG. 1 may also be implemented with avariety of fabrics and protocols. Examples of such fabrics includeEthernet (802.3), wireless (802.11), and the like. Examples of such datacommunications protocols include Transmission Control Protocol (‘TCP’),User Datagram Protocol (‘UDP’), Internet Protocol (‘IP’), HyperTextTransfer Protocol (‘HTTP’), Wireless Access Protocol (‘WAP’), HandheldDevice Transport Protocol (‘HDTP’), Real Time Protocol (‘RTP’) andothers as will occur to those of skill in the art.

The example storage arrays (102, 104) of FIG. 1 provide persistent datastorage for the computing devices (164, 166, 168, 170). Each storagearray (102, 104) depicted in FIG. 1 includes a storage array controller(106, 112). Each storage array controller (106, 112) may be embodied asa module of automated computing machinery comprising computer hardware,computer software, or a combination of computer hardware and software.The storage array controllers (106, 112) may be configured to carry outvarious storage-related tasks. Such tasks may include writing datareceived from the one or more of the computing devices (164, 166, 168,170) to storage, erasing data from storage, retrieving data from storageto provide the data to one or more of the computing devices (164, 166,168, 170), monitoring and reporting of disk utilization and performance,performing RAID (Redundant Array of Independent Drives) or RAID-likedata redundancy operations, compressing data, encrypting data, and soon.

Each storage array controller (106, 112) may be implemented in a varietyof ways, including as a Field Programmable Gate Array (‘FPGA’), aProgrammable Logic Chip (‘PLC’), an Application Specific IntegratedCircuit (‘ASIC’), or computing device that includes discrete componentssuch as a central processing unit, computer memory, and variousadapters. Each storage array controller (106, 112) may include, forexample, a data communications adapter configured to supportcommunications via the SAN (158) and the LAN (160). Although only one ofthe storage array controllers (112) in the example of FIG. 1 is depictedas being coupled to the LAN (160) for data communications, readers willappreciate that both storage array controllers (106, 112) may beindependently coupled to the LAN (160). Each storage array controller(106, 112) may also include, for example, an I/O controller or the likethat couples the storage array controller (106, 112) for datacommunications, through a midplane (114), to a number of storage devices(146, 150), and a number of write buffer devices (148, 152) that areutilized as write caches.

Each write buffer device (148, 152) may be configured to receive, fromthe storage array controller (106, 112), data to be stored in thestorage devices (146). Such data may originate from any one of thecomputing devices (164, 166, 168, 170). In the example of FIG. 1,writing data to the write buffer device (148, 152) may be carried outmore quickly than writing data to the storage device (146, 150). Thestorage array controller (106, 112) may be configured to effectivelyutilize the write buffer devices (148, 152) as a quickly accessiblebuffer for data destined to be written to storage. In this way, thelatency of write requests may be significantly improved relative to asystem in which the storage array controller writes data directly to thestorage devices (146, 150). In addition, the storage array controller(106, 112) can accumulate more writes and organize writing to thestorage devices (146, 150) for greater efficiency with bettercompression and with more time (and more and larger blocks ofaccumulated write data) for de-duplication. Such write buffer devices(148, 152) effectively convert storage arrays of solid-state drives(e.g., “Flash drives”) from latency limited devices to throughputlimited devices. In such a way, the storage array controller (106, 112)may be given more time to better organize what is written to the storagedevices (146, 150), but after doing so, are not then mechanicallylimited like disk-based arrays are.

A ‘storage device’ as the term is used in this specification refers toany device configured to record data persistently. The term‘persistently’ as used here refers to a device's ability to maintainrecorded data after loss of a power source. Examples of storage devicesmay include mechanical, spinning hard disk drives, solid-state drives,and the like.

The storage array controllers (106, 112) of FIG. 1 may be useful inmanaging I/O queues in a storage system according to embodiments of thepresent disclosure. The storage array controllers (106, 112) may assistin managing I/O queues in a data storage system by: receiving an I/Ooperation to be serviced by a target storage device; determining, foreach of a plurality of paths, a data transfer maximum associated withthe path; determining, for one or more of the plurality of paths, acumulative amount of data to be transferred by I/O operations pending onthe path; and selecting a target path for transmitting the I/O operationto the target storage device in dependence upon the cumulative amount ofdata to be transferred by I/O operations pending on the path and thedata transfer maximum associated with the path, and performing otherfunctions as will be described in greater detail below.

The arrangement of computing devices, storage arrays, networks, andother devices making up the example system illustrated in FIG. 1 are forexplanation, not for limitation. Systems useful according to variousembodiments of the present disclosure may include differentconfigurations of servers, routers, switches, computing devices, andnetwork architectures, not shown in FIG. 1, as will occur to those ofskill in the art.

Managing I/O queues in a data storage system in accordance withembodiments of the present disclosure is generally implemented withcomputers. In the system of FIG. 1, for example, all the computingdevices (164, 166, 168, 170) and storage controllers (106, 112) may beimplemented to some extent at least as computers. For furtherexplanation, therefore, FIG. 2 sets forth a block diagram of a storagearray controller (202) useful in managing I/O queues in a data storagesystem according to embodiments of the present disclosure.

The storage array controller (202) of FIG. 2 is similar to the storagearray controllers depicted in FIG. 1, as the storage array controller(202) of FIG. 2 is communicatively coupled, via a midplane (206), to oneor more storage devices (212) and to one or more write buffer devices(214) that are included as part of a storage array (216). The storagearray controller (202) may be coupled to the midplane (206) via one ormore data communications links (204) and the midplane (206) may becoupled to the storage devices (212) and the memory buffer devices (214)via one or more data communications links (208, 210). The datacommunications links (204, 208, 210) of FIG. 2 may be embodied, forexample, as Peripheral Component Interconnect Express (‘PCIe’) bus.

The storage array controller (202) of FIG. 2 includes at least onecomputer processor (232) or ‘CPU’ as well as random access memory (RAM′)(236). The computer processor (232) may be connected to the RAM (236)via a data communications link (230), which may be embodied as a highspeed memory bus such as a Double-Data Rate 4 (‘DDR4’) bus.

Stored in RAM (236) is an operating system (246). Examples of operatingsystems useful in storage array controllers (202) configured formanaging I/O queues in a data storage system according to embodiments ofthe present disclosure include UNIX™, Linux™, Microsoft Windows′, andothers as will occur to those of skill in the art. Also stored in RAM(236) is a queue management module (248), a module that includescomputer program instructions useful in managing I/O queues in a datastorage system according to embodiments of the present disclosure.

The queue management module (248) may compress data in dependence uponcharacteristics of a storage system by: receiving an I/O operation to beserviced by a target storage device; determining, for each of aplurality of paths between the host and the target storage device, adata transfer maximum associated with the path, wherein the datatransfer maximum specifies a cumulative amount of data that may beassociated with I/O operations pending on the path; determining, for oneor more of the plurality of paths, a cumulative amount of data to betransferred by I/O operations pending on the path; and selecting atarget path for transmitting the I/O operation to the target storagedevice in dependence upon the cumulative amount of data to betransferred by I/O operations pending on the path and the data transfermaximum associated with the path, as will be described in greater detailbelow.

The queue management module (248) may further managing I/O queues in adata storage system by: selecting a target path in further dependenceupon the amount of data to be transferred when executing the one or moreI/O operations; determining, for each of the plurality of paths, whethera sum of the amount of data to be transferred when executing the I/Ooperation and the cumulative amount of data to be transferred by I/Ooperations pending on the path is greater than the data transfer maximumassociated with the path; responsive to determining, for each of theplurality of paths, that the sum of the amount of data to be transferredwhen executing the I/O operation and the cumulative amount of data to betransferred by I/O operations pending on the path is greater than thedata transfer maximum associated with the path, delaying a selection ofthe target path responsive to determining, for each of the plurality ofpaths, that the sum of the amount of data to be transferred whenexecuting the I/O operation and the cumulative amount of data to betransferred by I/O operations pending on the path is greater than thedata transfer maximum associated with the path; determining whether anamount of data associated with the I/O operation is less than apredetermined size; responsive to determining that the amount of dataassociated with the I/O operation is less than the predetermined size,selecting the target path for transmitting the I/O operation to thestorage device in further dependence upon a number of I/O operationspending on the path; tuning the data transfer maximum associated withone or more paths, including: modifying, for one or more of theplurality of paths, the data transfer maximum associated with the path;determining, in dependence upon one or more performance metrics, whetherpath performance has improved after the data transfer maximum associatedwith the path was modified; and responsive to determining that pathperformance has not improved after the data transfer maximum associatedwith the path was modified, reverting the data transfer maximumassociated with the path to its previous state; and responsive todetermining that path performance has improved after the data transfermaximum associated with the path was modified, modifying the datatransfer maximum associated with the path, as will be described ingreater detail below.

The storage array controller (202) of FIG. 2 also includes a pluralityof host bus adapters (218, 220, 222) that are coupled to the processor(232) via a data communications link (224, 226, 228). Each host busadapter (218, 220, 222) may be embodied as a module of computer hardwarethat connects the host system (i.e., the storage array controller) toother network and storage devices. Each of the host bus adapters (218,220, 222) of FIG. 2 may be embodied, for example, as a Fibre Channeladapter that enables the storage array controller (202) to connect to aSAN, as an Ethernet adapter that enables the storage array controller(202) to connect to a LAN, as a Target Channel Adapter, as aSCSI/Storage Target Adapter, and so on. Each of the host bus adapters(218, 220, 222) may be coupled to the computer processor (232) via adata communications link (224, 226, 228) such as, for example, a PCIebus.

The storage array controller (202) of FIG. 2 also includes a switch(244) that is coupled to the computer processor (232) via a datacommunications link (238). The switch (244) of FIG. 2 may be embodied asa computer hardware device that can create multiple endpoints out of asingle endpoint, thereby enabling multiple devices to share what wasinitially a single endpoint. The switch (244) of FIG. 2 may be embodied,for example, as a PCIe switch that is coupled to a PCIe bus (238) andpresents multiple PCIe connection points to the midplane (206).

The storage array controller (202) of FIG. 2 may also include a datacommunications link (234) for coupling the storage array controller(202) to other storage array controllers. Such a data communicationslink (234) may be embodied, for example, as a QuickPath Interconnect(‘QPI’) interconnect, as PCIe non-transparent bridge (‘NTB’)interconnect, and so on.

Readers will recognize that these components, protocols, adapters, andarchitectures are for illustration only, not limitation. Such a storagearray controller may be implemented in a variety of different ways, eachof which is well within the scope of the present disclosure.

For further explanation, FIG. 3 a block diagram of a storage systemconfigured for managing I/O queues according to embodiments of thepresent disclosure. The example storage system includes a host (302) anda storage array (324) that are coupled for data communications by twodata communication links (326, 328). The data communication links (326,328) may be embodied, for example, as SCSI bus or other appropriate datacommunications bus.

The host (302) depicted in FIG. 3 includes elements of an I/O stack suchas an application (304) and a file system (306). The application (304)may be embodied as a software application executing on computer hardwaresuch as a computer processor. The application (304) can sit on top ofthe file system (306), although such an application could also sit ontop of a database or an operating system running on a hypervisor), whilethe file system (306) may sit on top of local components that facilitateaccess to the storage array (324) such as multipathing module (308). Themultipathing module (308) may be embodied, for example, as an instanceof a Linux multipathing device mapper driver.

The host depicted in FIG. 3 also includes two initiators (310, 312). Thetwo initiators (310, 312) depicted in FIG. 3 may be embodied as a datacommunications hardware utilized to communicate with the storage array(324) and to direct access requests (e.g., read, write, create, delete)to the storage array (324). The two initiators (310, 312) may beembodied, for example, as a SCSI host bus adapter, an iSCSI host busadapter, a Fibre-Channel host bus adapter, and so on.

The storage array depicted in FIG. 3 also includes two targets (314,316). The two targets (314, 316) depicted in FIG. 3 may be embodied as adata communications hardware utilized to communicate with the host (302)and to respond to access requests (e.g., read, write, create, delete)received from the host (302). The two targets (314, 316) may beembodied, for example, as a SCSI host bus adapter, an iSCSI host busadapter, a Fibre-Channel host bus adapter, as a basic Ethernet NetworkInterface Card with a software implementation of iSCSI or Fibre-Channelover Ethernet, and so on.

The storage array (324) depicted in FIG. 3 includes storage devices(322), internal busses (320), and array logic (318). The storage devices(322) may be embodied, for example, as solid-state drives (‘SSDs’) thatare coupled to the two targets (314, 316) by internal busses (320) andarray logic (318) that can direct access requests to the appropriatestorage device (322). Readers will appreciate that in the exampledepicted in FIG. 3, multiple paths may exist between the host (302) andeach of the storage devices (322) in the storage array (324). Forexample, a first path may run through a first initiator (310) and afirst target (314) while a second path may run through a secondinitiator (312) and a second target (316).

For further explanation, FIG. 4 sets forth a flow chart illustrating anexample method for managing I/O queues in a data storage system (404)according to embodiments of the present disclosure. Although depicted inless detail, the data storage system (404) of FIG. 4 may be similar tothe storage arrays described above with reference to FIG. 1, as the datastorage system (404) may include multiple storage arrays, multiplestorage array controllers in each storage array, and multiple storagedevices in each storage array.

The data storage system (404) depicted in FIG. 4 includes a host (406)that is coupled to a plurality of storage devices (424, 426, 428) via astorage network (422). The host (406) in FIG. 4 may be embodied, forexample, as a storage array controller that is similar to the storagearray controller described above with reference to FIG. 2 and alsosimilar to the host depicted in FIG. 3. The storage network (422) may beembodied, for example, as a group of data communications links and datacommunications hardware that are used for data communications betweenthe host (406) and the plurality of storage devices (424, 426, 428).

The storage network (422) depicted in FIG. 4 may include multiple pathsbetween each storage device (424, 426, 428) and the host (406). Themultiple paths between each storage device (424, 426, 428) and the host(406) may be created, for example, due to the presence of multiple datacommunications paths through a network, due to the presence of multiplesoftware modules that are involved in the exchange of data between aparticular storage device (424, 426, 428) and the host (406), due to thepresence of multiple data communications adapters on a particularstorage device (424, 426, 428) or the host (406), for other reasons aswill occur to those of skill in the art in view of the teachingscontained herein, or as the result of any combination of such factors.

Each of the multiple paths between each storage device (424, 426, 428)and the host (406) may include one or more I/O queues. Each I/O queuecan be included, for example, as part of a host bus adapter driver thatmanages a bus that is part of a path between each storage device (424,426, 428) and the host (406), as part of an operating system SCSI devicedriver that manages a SCSI device such as the storage device (424, 426,428), as part of multipathing software that manages the flow of I/Ooperations between the host (406) and each storage device (424, 426,428), and so on. When any of these I/O queues get too full, total systemperformance may lag due to path imbalances resulting in some pathsbecoming idle while another is still busy processing a large I/O queue.

In the example method depicted in FIG. 4, the I/O operations that areexchanged between the host (406) and each storage device (424, 426, 428)may include I/O operations of different types. Read operations and writeoperations are one example of I/O operations that are of differenttypes. In the example method depicted in FIG. 4, each distinct path maybe utilized to service I/O operations of a particular type between thehost (406) and a particular target storage device (424, 426, 428). Forexample, a first path may be utilized to transmit data read from the afirst storage device (424) to the host (406) in response to a readoperation, a second path may be utilized to transmit data to be writtento the first storage device (424) from the host (406) in response to awrite operation, a third path may be utilized to transmit data read froma second storage device (426) to the host (406) as part of a readoperation, and so on. Readers will appreciate that although exampledescribed above relates to embodiments where multiple paths are used toservice different types of I/O operations between the host (406) and aparticular target storage device (424, 426, 428), in alternativeembodiments of the present disclosure, the distinct paths that are usedto service I/O operations of a particular type may be distinctdirections within a multi-directional path. For example, each path mayhave both a transmit-from-host-to-storage direction and atransmit-from-storage-to-host direction. Requests and write data movefrom host to storage. Read data and status/completion responses movefrom storage to host. These paths may be “full duplex,” meaning that thefull bandwidth is available in each direction. As a result, therequest/response overhead is shared between all I/O types, while writedata transfer bandwidth (e.g., the 128K part of a 128K write) uses onlythe host-to-storage bandwidth, and while read data transfer bandwidth(e.g., the 128K part of a 128K read) uses only storage-to-hostbandwidth.

The example method depicted in FIG. 4 can include receiving (408) an I/Ooperation (402) to be serviced by a target storage device (424, 426,428). The I/O operation (402) to be serviced by a target storage device(424, 426, 428) can include, for example, a read operation, a writeoperation, or other I/O operation. The I/O operation (402) that is to beserviced by a target storage device (424, 426, 428) may be received(408) by the host (406), for example, from a computer that is connectedto the host (406) via a storage area network, such as the computers thatare connected to the storage array controllers in FIG. 1 via a storagearea network.

The example method depicted in FIG. 4 can also include determining(410), for each of a plurality of paths between the host (406) and thetarget storage device (424, 426, 428), a data transfer maximum (412)associated with the path. The data transfer maximum (412) for each pathcan specify a cumulative amount of data that is allowed to be associatedwith I/O operations pending on the path. An I/O operations is said to be‘pending’ on a particular path when the I/O operation has been sent fromthe host (406) to a particular storage device (424, 426, 428) via thepath, but the I/O operation has not yet completed as the host (406) hasnot received an acknowledgement of completion from the storage device(424, 426, 428). Data is associated with an I/O operation that ispending on the path, for example, if the data is to be written to astorage device (424, 426, 428) as the result of executing the I/Ooperation, if the data is to be read from a storage device (424, 426,428) as the result of executing the I/O operation, and so on.

In the example method depicted in FIG. 4, determining (410) a datatransfer maximum (412) associated with each path may be carried out bythe host (406) examining a table, database, or other data store thatassociates path identifiers with the data transfer maximum (412) for thepath that is associated with the path identifier. The values containedin such a table, database, or other data store may be originallyconfigured to preset values during system startup and may be dynamicallytuned when the system is operating, as will be described in greaterdetail below.

The example method depicted in FIG. 4 can also include determining(414), for one or more of the plurality of paths, a cumulative amount ofdata (416) to be transferred by I/O operations pending on the path. Thecumulative amount of data (416) to be transferred by I/O operationspending on the path can be determined (414), for example, by identifyingall I/O operations pending on a particular path and summing the amountof data that is associated with each of the I/O operations pending onthe particular path. Consider an example in which five I/O operations towrite data to a particular storage device (428) are pending on aparticular path between the storage device (428) and the host (406): 1)a first I/O operation to write 128 KB of data to the storage device(428), 2) a second I/O operation to write 64 KB of data to the storagedevice (428), 3) a third I/O operation to write 16 KB of data to thestorage device (428), 4) a fourth I/O operation to write 256 KB of datato the storage device (428), and 5) a fifth I/O operation to write 32 KBof data to the storage device (428). In such an example, the cumulativeamount of data (416) to be transferred by I/O operations pending on thepath would be equal to (128 KB+64 KB+16 KB+256 KB+32 KB) 496 KB.

The example method depicted in FIG. 4 can also include selecting (418) atarget path (420) for transmitting the I/O operation (402) to the targetstorage device in dependence upon the cumulative amount of data (416) tobe transferred by I/O operations pending on each path and the datatransfer maximum (412) associated with each path. Selecting (418) atarget path (420) for transmitting the I/O operation (402) to the targetstorage device in dependence upon the cumulative amount of data (416) tobe transferred by I/O operations pending on each path and the datatransfer maximum (412) associated with each path may be carried out, forexample, by selecting (418) a target path (420) whose data transfermaximum (412) is greater than the cumulative amount of data (416) to betransferred by I/O operations pending on the path, by selecting (418) atarget path (420) whose data transfer maximum (412) is greater than thecumulative amount of data (416) to be transferred by I/O operationspending on the path by a predetermined threshold, by selecting (418) atarget path (420) with the largest differential between its datatransfer maximum (412) and the cumulative amount of data (416) to betransferred by I/O operations pending on the path, and so on. In someembodiments, the size of data associated with the I/O operation may betaken into account, such that a target path (420) whose data transfermaximum (412) is greater than the sum of the size of data associatedwith the I/O operation and the cumulative amount of data (416) to betransferred by I/O operations pending on the path is selected (418).

Consider an example in which three candidate target paths are available,where the first path has a data transfer maximum of 512 KB, the secondfirst path has a data transfer maximum of 1024 KB, and the third pathhas a data transfer maximum of 768 KB. In such an example, assume thatthe cumulative amount of data to be transferred by I/O operationspending on the first path is 256 KB, the cumulative amount of data to betransferred by I/O operations pending on the second path is 1024 KB, andthe cumulative amount of data to be transferred by I/O operationspending on the third path is 640 KB. In such an example, the second pathwould not be selected (418) as the target path (420) given that thecumulative amount of data to be transferred by I/O operations pending onthe second path has already reached the data transfer maximum of thesecond path. The first path and third path, however, would be availablefor selection (418) as the target path (420).

Continuing with the example described in the preceding paragraph, insome embodiments of the present invention the first path, the thirdpath, or both paths may also be excluded from being selected (418) asthe target path (420). Consider an example in which the I/O operationwas an instruction to write 192 KB of data. In such an example, inembodiments where the size of the I/O instruction is also taken intoaccount, the third path would not be selected (418) as the target path(420) given that the cumulative amount of data to be transferred by I/Ooperations pending on the third path is only 128 KB less than the datatransfer maximum of the third path. The first path, however, could beselected (418) as the target path (420) given that the cumulative amountof data to be transferred by I/O operations pending on the first path is256 KB less than the data transfer maximum of the first path. Readerswill appreciate that the path selection described above can be carriedout through the use of a calculation that takes into account parametersof the new I/O operation and the potential paths, including at least thesize of the new I/O operation and the cumulative I/O transfer sizespending for the path.

For further explanation, FIG. 5 sets forth a flow chart illustrating anadditional example method for managing I/O queues in a data storagesystem (404) according to embodiments of the present disclosure. Theexample method depicted in FIG. 5 is similar to the example methoddepicted in FIG. 4, as the example method depicted in FIG. 5 alsoincludes receiving (408) an I/O operation (402) to be serviced by atarget storage device (424, 426, 428), determining (410) a data transfermaximum (412) associated with each of a plurality of paths between thehost (406) and the target storage device (424, 426, 428), determining(414) a cumulative amount of data (416) to be transferred by I/Ooperations pending on one or more of the plurality of paths, andselecting (418) a target path (420) for transmitting the I/O operation(402) to the target storage device in dependence upon the cumulativeamount of data (416) to be transferred by I/O operations pending on eachpath and the data transfer maximum (412) associated with each path.

The example method depicted in FIG. 5 also includes determining (502),for each of the plurality of paths, whether a sum of the amount of datato be transferred when executing the I/O operation (402) and thecumulative amount of data (416) to be transferred by I/O operationspending on the path is greater than the data transfer maximum (412)associated with the path. If the host (406) determines that the sum ofthe amount of data to be transferred when executing the I/O operation(402) and the cumulative amount of data (416) to be transferred by I/Ooperations pending on each path is not (508) greater than the datatransfer maximum (412) associated with each path, the host may proceedto select (418) a target path (420). If, however, the host (406)affirmatively (504) determines that the sum of the amount of data to betransferred when executing the I/O operation (402) and the cumulativeamount of data (416) to be transferred by I/O operations pending on eachpath is greater than the data transfer maximum (412) associated witheach path, the host (406) may not be able to select (418) a target path(420) immediately.

The example method depicted in FIG. 5 also includes delaying (506) aselection of the target path (420). In the example method depicted inFIG. 5, delaying (506) a selection of the target path (420) is carriedout in response to affirmatively (504) determining that the sum of theamount of data to be transferred when executing the I/O operation (402)and the cumulative amount of data (416) to be transferred by I/Ooperations pending on each path is greater than the data transfermaximum (412) associated with each path. Delaying (506) a selection ofthe target path (420) may be carried out, for example, by the host (406)waiting a predetermined period of time and then determining (414) anupdated cumulative amount of data (416) to be transferred by I/Ooperations pending on one or more of the plurality of paths, by the host(406) waiting until an acknowledgment is received indicating that apredetermined number of pending I/O operations have completed and thendetermining (414) an updated cumulative amount of data (416) to betransferred by I/O operations pending on one or more of the plurality ofpaths, and so on. Readers will appreciate that in embodiments of thepresent disclosure, delaying (506) a selection of the target path (420)may also be carried out through the use of functions that are includedwithin the I/O stack. For example, when an I/O completes, a functionwithin the driver stack is called to note the completion. Such afunction can recalculate the new cumulative amount of data (416) to betransferred by I/O operations pending on each path, such that theselection of a target path (420) is not delayed a predetermined periodof time but rather delayed until the function naturally detects that apath is available for receiving pending requests.

The preceding paragraph identifies variables (e.g., the predeterminedperiod of time, the predetermined number of pending I/O operations thathave completed) that are used to determine how long the selection of thetarget path (420) is delayed (506). Readers will appreciate that suchvariables may be managed as boot-time parameters whose values are setduring startup of the host (406). Readers will appreciate that suchvariables may be tuned over time by changing the value of such variablesand analyzing whether system performance improved, held steady, ordegraded in response to changing the values of such variables. Thevalues of such variables may be tuned in a manner that is similar to themanner in which queue sizes are tuned, as described below with referenceto FIG. 7. For example, the host (406) may continue to adjust the valueof the variables in the same direction (i.e., increasing or decreasing)as changes that caused system performance to improve, the host (406) mayback out changes that caused system performance to degrade or holdsteady, and so on.

Readers will further appreciate that although embodiments are describedabove where delaying (506) a selection of the target path (420) iscarried out in response to affirmatively (504) determining that the sumof the amount of data to be transferred when executing the I/O operation(402) and the cumulative amount of data (416) to be transferred by I/Ooperations pending on each path is greater than the data transfermaximum (412) associated with each path, in alternative embodimentsdelaying (506) a selection of the target path (420) may only occur oncethe cumulative amount of data (416) to be transferred by I/O operationspending on each path is greater than the data transfer maximum (412)associated with each path. In such an embodiment, even if placing theI/O operation (402) on a particular path would cause the cumulativeamount of data (416) to be transferred by all I/O operations pending onthe particular path to exceed the data transfer maximum (412) associatedwith the particular path, the I/O operation (402) may still be placed onthe particular path. In such a way, delaying (506) a selection of thetarget path (420) may be carried out only when all paths are actuallyoversubscribed rather than at the point where introducing additional I/Ooperations to a path would cause all of the I/O paths to becomeoversubscribed.

In the example method depicted in FIG. 5, selecting (418) the targetpath (420) can include selecting (510) a target path (420) in furtherdependence upon the amount of data to be transferred when executing theI/O operation (402). Selecting (510) a target path (420) in furtherdependence upon the amount of data to be transferred when executing theI/O operation (402) may be carried out, for example, by adding theamount of data to be transferred when executing the I/O operation (402)to the cumulative amount of data (416) to be transferred by I/Ooperations pending a particular path, to identify the amount of datathat would be pending on the path if the path were selected (418) as thetarget path (420). In such an example, the amount of data that would bepending on the path if the path were selected (418) as the target path(420) may be compared to the data transfer maximum (412) associated withthe path to determine whether the path has sufficient bandwidthavailable to serve as the target path (420).

Consider an example in which three candidate target paths are available,where the first path has a data transfer maximum of 512 KB, the secondfirst path has a data transfer maximum of 1024 KB, and the third pathhas a data transfer maximum of 768 KB. In such an example, assume thatthe cumulative amount of data to be transferred by I/O operationspending on the first path is 256 KB, the cumulative amount of data to betransferred by I/O operations pending on the second path is 1024 KB, andthe cumulative amount of data to be transferred by I/O operationspending on the third path is 640 KB. Further assume that the I/Ooperation was an instruction to write 192 KB of data. In such anexample, the second path would not be selected (418) as the target path(420) given that the sum of the amount of data to be transferred whenexecuting the I/O operation (402) and the cumulative amount of data(416) to be transferred by I/O operations pending the second pathexceeds the data transfer maximum (412) for the second path. The thirdpath would also not be selected (418) as the target path (420) giventhat the sum of the amount of data to be transferred when executing theI/O operation (402) and the cumulative amount of data (416) to betransferred by I/O operations pending the third path exceeds the datatransfer maximum (412) for the third path The first path, however, couldbe selected (418) as the target path (420) given that the cumulativeamount of data to be transferred by I/O operations pending on the firstpath is 256 KB less than the data transfer maximum of the first path.

For further explanation, FIG. 6 sets forth a flow chart illustrating anadditional example method for managing I/O queues in a data storagesystem (404) according to embodiments of the present disclosure. Theexample method depicted in FIG. 6 is similar to the example methoddepicted in FIG. 4, as the example method depicted in FIG. 6 alsoincludes receiving (408) an I/O operation (402) to be serviced by atarget storage device (424, 426, 428), determining (410) a data transfermaximum (412) associated with each of a plurality of paths between thehost (406) and the target storage device (424, 426, 428), determining(414) a cumulative amount of data (416) to be transferred by I/Ooperations pending on one or more of the plurality of paths, andselecting (418) a target path (420) for transmitting the I/O operation(402) to the target storage device in dependence upon the cumulativeamount of data (416) to be transferred by I/O operations pending on eachpath and the data transfer maximum (412) associated with each path.

The example method depicted in FIG. 6 includes determining (602), forthe I/O operation (402), whether an amount of data associated with theI/O operation (402) is less than a predetermined size. The predeterminedsize may represent a threshold that is used to distinguish between largeI/O operations and small I/O operations. Distinguish between large I/Ooperations and small I/O operations may be useful because performing adetailed analysis of which path should be utilized to transfer aparticular I/O operation between the host (406) and a storage device(424, 426, 428) requires the utilization of processing resources. Assuch, the benefit to be gained by strategically selecting a path for asmall I/O operation may not be justified by the cost of consuming suchprocessing resources to perform the selection. The benefit to be gainedby strategically selecting a path for a large I/O operation, however,may be justified by the cost of consuming such processing resources toperform the selection.

Readers will appreciate that the predetermined size that is utilized asa threshold may be set at a predetermined value when the host (406) isinitially booted. The predetermined size, however, need not remainstatic. In fact, the predetermined size may be dynamically tuned byadjusting the predetermined size and determining whether such anadjustment resulted in improved system performance, degraded systemperformance, or no change to system performance. Such tuning will bedescribed in greater detail below in the context of tuning the datatransfer maximum (412) that is associated with each path, but readerswill appreciate that similar principles may be utilized to tune thepredetermined size that is utilized as a threshold for distinguishingbetween large I/O operations and small I/O operations. Readers willfurther appreciate that the predetermined size that is utilized as athreshold for distinguishing between large I/O operations and small I/Ooperations may be different for different types of I/O operations.

In the example method depicted in FIG. 6, when the host (406) determinesthat amount of data associated with the I/O operation (402) is not (604)less than a predetermined size, the host (406) may proceed withdetermining (410) a data transfer maximum (412) associated with each ofa plurality of paths between the host (406) and the target storagedevice (424, 426, 428), determining (414) a cumulative amount of data(416) to be transferred by I/O operations pending on one or more of theplurality of paths, and selecting (418) a target path (420) fortransmitting the I/O operation (402) to the target storage device independence upon the cumulative amount of data (416) to be transferred byI/O operations pending on each path and the data transfer maximum (412)associated with each path, as the I/O operation (402) is a large I/Ooperation. When the host (406) affirmatively (606) determines thatamount of data associated with the I/O operation (402) is less than apredetermined size, however, the host (406) may utilize the number ofI/O operations that are be pending on each particular path to drive theselection of a path.

The example method depicted in FIG. 6 also includes selecting (608) thetarget path (420) for transmitting the I/O operation (402) to thestorage device (424, 426, 428) in dependence upon a number of I/Ooperations pending on the path. Selecting (608) the target path (420)for transmitting the I/O operation (402) to the storage device (424,426, 428) in further dependence upon a number of I/O operations pendingon the path may be carried out in response to affirmatively (606)determining that the amount of data associated with the I/O operation isless than the predetermined size. Readers will appreciate that inaddition to each path having an associated data transfer maximum (412),each path may also have a maximum number of I/O operations that may bepending on the path. The host (406) may therefore track the number ofI/O operations that are pending on each path and may select (608) thetarget path (420) for transmitting the I/O operation (402) to thestorage device (424, 426, 428) in dependence upon a number of I/Ooperations pending on the path, for example, by selecting the path thathas the fewest number of pending I/O operations, by selecting the paththat has the lowest percentage of pending I/O operations relative to itscapacity, and so on. Readers will appreciate that once the path isselected, however, the size of the small I/O operation may still beincluded in the cumulative amount of data (416) to be transferred by I/Ooperations that are pending on the selected path.

Although not depicted in FIG. 6, readers will appreciate that theselection (608) of the target path (420) for transmitting the I/Ooperation (402) to the storage device (424, 426, 428) in dependence upona number of I/O operations pending on the path may be carried out inconjunction with selecting (418) a target path (420) for transmittingthe I/O operation (402) to the target storage device in dependence uponthe cumulative amount of data (416) to be transferred by I/O operationspending on each path and the data transfer maximum (412) associated witheach path. For example, the only paths that may be eligible forselection (608) as the target path (420) may be those paths for whichthe cumulative amount of data (416) to be transferred by I/O operationspending on the path is less than the data transfer maximum (412)associated with each path. Stated differently, any path for which thecumulative amount of data (416) to be transferred by I/O operationspending on the path is not less than the data transfer maximum (412)associated with the path may be ineligible for selection (608) as thetarget path (420) even if the number of I/O operations pending on thepath is well below allowable limits.

For further explanation, FIG. 7 sets forth a flow chart illustrating anadditional example method for managing I/O queues in a data storagesystem (404) according to embodiments of the present disclosure. Theexample method depicted in FIG. 7 is similar to the example methoddepicted in FIG. 4, as the example method depicted in FIG. 7 alsoincludes receiving (408) an I/O operation (402) to be serviced by atarget storage device (424, 426, 428), determining (410) a data transfermaximum (412) associated with each of a plurality of paths between thehost (406) and the target storage device (424, 426, 428), determining(414) a cumulative amount of data (416) to be transferred by I/Ooperations pending on one or more of the plurality of paths, andselecting (418) a target path (420) for transmitting the I/O operation(402) to the target storage device in dependence upon the cumulativeamount of data (416) to be transferred by I/O operations pending on eachpath and the data transfer maximum (412) associated with each path.

The example method depicted in FIG. 7 also includes tuning (702) thedata transfer maximum (412) associated with one or more paths. Tuning(702) the data transfer maximum (412) associated with one or more pathsmay be carried out, for example, by adjusting the data transfer maximum(412) for a particular path and determining whether system performanceimproves, degrades, or is unaffected by the adjusting the data transfermaximum (412). In such an example, the data transfer maximum (412) maycontinue to be adjusted in the same direction so long as systemperformance improves.

Consider the example described above in which a particular path had adata transfer maximum (412) of 256 KB. In such an example, tuning (702)the data transfer maximum (412) associated with such a path may becarried out, for example, by increasing the data transfer maximum (412)by 32 KB, such that the new data transfer maximum (412) for the path was288 KB. If system performance improves, the data transfer maximum (412)may be increase another 32 KB, such that the new data transfer maximum(412) for the path is 320 KB. If system performance again improves, thedata transfer maximum (412) may be increase another 32 KB, such that thenew data transfer maximum (412) for the path is 352 KB. This pattern maybe continued until, eventually, system performance degrades in responseto the increase. In such an example, the data transfer maximum (412) maybe rolled back 32 KB to the last value that resulted in improved systemperformance. In an example, the data transfer maximum (412) may keepincreasing and increasing until an optimal point is reached. Readerswill appreciate, however, that such tuning (702) may occur many timesduring the lifetime of the data storage system (404) such that the datatransfer maximum (412) for each path is periodically tuned. Readers willfurther appreciate that such tuning may also be carried out bydecreasing the data transfer maximum (412) to determine whether systemperformance improves in response to a decreased data transfer maximum(412).

In the example method depicted in FIG. 7, tuning (702) the data transfermaximum (412) associated with one or more paths can include modifying(704) the data transfer maximum (412) associated with the path.Modifying (704) the data transfer maximum (412) associated with the pathmay be carried out, for example, by decreasing the data transfer maximum(412) associated with the path by a predetermined amount, decreasing thedata transfer maximum (412) associated with the path by a predeterminedpercentage, increasing the data transfer maximum (412) associated withthe path by a predetermined amount, increasing the data transfer maximum(412) associated with the path by a predetermined percentage, and so on.In such an example, the new data transfer maximum (412) associated withthe path may be associated in a table, database, or other datarepository that contains the data transfer maximum (412) associated withthe path.

In the example method depicted in FIG. 7, tuning (702) the data transfermaximum (412) associated with one or more paths can also includedetermining (706) path performance for a period of time after the datatransfer maximum (412) associated with the path was modified (704). Thepath performance can be expressed, for example, as the average latencyfor I/O operations, as the average latency for I/O operations ofdifferent types, as the average IOPS performed, as the average IOPS of aparticular type that are performed, as the average I/O latency througheach path, as the average IOPS performed through each path, and so on.In the example depicted in FIG. 7, the host (406) may maintain orotherwise have access to historical performance metrics for the datastorage system (404). The host (406) may therefore retrieve theperformance metrics for the data storage system (404) that weregenerated when the data storage system (404) was utilizing theimmediately preceding data transfer maximum (412) associated with thepath. The host (406) may subsequently compare the performance metricsfor the data storage system (404) that were generated when the datastorage system (404) was utilizing the immediately preceding datatransfer maximum (412) associated with the path to performance metricsthat were generated after the data transfer maximum (412) associatedwith the path was modified (704).

The example method depicted in FIG. 7 also includes determining (707)whether path performance meets a threshold after the data transfermaximum (412) associated with the path was modified (704). In responseto determining that path performance does not (708) meet the thresholdafter the data transfer maximum (412) associated with the path wasmodified, the host (406) may be configured to revert (710) the datatransfer maximum (412) associated with the path to its previous state.In the example method depicted in FIG. 7, the threshold may be embodiedas a dynamic value that may change with each change to the data transfermaximum (412) associated with the path. Consider a first example inwhich the data transfer maximum (412) associated with the path wasmodified by increasing the data transfer maximum (412). In such anexample, the threshold may be set to a value that is greater than thepath performance that was achieved when the data transfer maximum (412)associated with the path was in its previous state. In such a way, thedata transfer maximum (412) associated with the path may be reverted(710) to its previous state when increasing the data transfer maximum(412) associated with the path results in path performance degrading orpath performance failing to materially change. In a second example inwhich the data transfer maximum (412) associated with the path wasmodified by decreasing the data transfer maximum (412), however, thethreshold may be set to a value that is equal to the path performancethat was achieved when the data transfer maximum (412) associated withthe path was in its previous state. In such a way, the data transfermaximum (412) associated with the path may be reverted (710) to itsprevious state only when decreasing the data transfer maximum (412)associated with the path results in path performance degrading, whereaspath performance failing to materially change will not result in thedata transfer maximum (412) associated with the path being reverted(710) to its previous state.

In the example method depicted in FIG. 7, tuning (702) the data transfermaximum (412) associated with one or more paths can also include againmodifying (704) the data transfer maximum (412) associated with thepath. Modifying (704) the data transfer maximum (412) associated withthe path may be carried out in response to affirmatively (712)determining that path performance meets the threshold after the datatransfer maximum (412) associated with the path was previously modified.In the example method depicted in FIG. 7, again modifying (704) the datatransfer maximum (412) associated with the path may be carried out bymodifying the data transfer maximum (412) in the same direction thatresulted in the threshold being met. For example, an increase to thedata transfer maximum (412) that resulted in improved system performancemay be followed by additional increases to the data transfer maximum(412), until one of the increases does not result in improved systemperformance, at which point the increase is reverted. Likewise, adecrease to the data transfer maximum (412) that resulted in improvedsystem performance or no material change to system performance may befollowed by additional decreases to the data transfer maximum (412),until one of the decrease results in degraded system performance, atwhich point the decrease is reverted.

Readers will appreciate that although the embodiments described aboverelate to embodiments where a single I/O operation (402) is evaluatedand ultimately sent to a storage device via a selected path, multipleI/O operations may be pooled and processed in the same way. Furthermore,although embodiments are described above where a target path (420) fortransmitting the I/O operation (402) to the target storage device isselected (418) in dependence upon only the cumulative amount of data(416) to be transferred by I/O operations pending on each path and thedata transfer maximum (412) associated with each path, readers willappreciate that the usage of the cumulative amount of data (416) to betransferred by I/O operations pending on each path and the data transfermaximum (412) associated with each path may be included in part of atarget path (420) selection scheme that takes into account many otherparameters such as queue depth or any other parameter.

Example embodiments of the present invention are described largely inthe context of a fully functional computer system for managing I/Oqueues in a data storage system. Readers of skill in the art willrecognize, however, that the present invention also may be embodied in acomputer program product disposed upon computer readable storage mediafor use with any suitable data processing system. Such computer readablestorage media may be any storage medium for machine-readableinformation, including magnetic media, optical media, or other suitablemedia. Examples of such media include magnetic disks in hard drives ordiskettes, compact disks for optical drives, magnetic tape, and othersas will occur to those of skill in the art. Persons skilled in the artwill immediately recognize that any computer system having suitableprogramming means will be capable of executing the steps of the methodof the invention as embodied in a computer program product. Personsskilled in the art will recognize also that, although some of theexample embodiments described in this specification are oriented tosoftware installed and executing on computer hardware, nevertheless,alternative embodiments implemented as firmware or as hardware are wellwithin the scope of the present invention.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Readers will appreciate that the steps described herein may be carriedout in a variety ways and that no particular ordering is required. Itwill be further understood from the foregoing description thatmodifications and changes may be made in various embodiments of thepresent invention without departing from its true spirit. Thedescriptions in this specification are for purposes of illustration onlyand are not to be construed in a limiting sense. The scope of thepresent invention is limited only by the language of the followingclaims.

What is claimed is:
 1. A method of dynamic path selection in a storagenetwork, the method comprising: receiving, by a host that is coupled toa plurality of storage devices via a storage network, an I/O operationto be serviced by a target storage device; determining, for each of aplurality of paths between the host and the target storage device, adata transfer maximum associated with the path, wherein the datatransfer maximum specifies a cumulative amount of data that may beassociated with I/O operations pending on the path; determining, for oneor more of the plurality of paths, a number of I/O operations pending onthe path; and selecting a target path for transmitting the I/O operationto the target storage device in dependence upon the number of I/Ooperations pending on the path and the data transfer maximum associatedwith the path.
 2. The method of claim 1 wherein selecting the targetpath further comprises selecting a target path in further dependenceupon the amount of data to be transferred when executing the I/Ooperation.
 3. The method of claim 1 further comprising: determining, foreach of the plurality of paths, whether a sum of the amount of data tobe transferred when executing the I/O operation and the cumulativeamount of data to be transferred by I/O operations pending on the pathis greater than the data transfer maximum associated with the path; andresponsive to determining, for each of the plurality of paths, that thesum of the amount of data to be transferred when executing the I/Ooperation and the cumulative amount of data to be transferred by I/Ooperations pending on the path is greater than the data transfer maximumassociated with the path, delaying a selection of the target path. 4.The method of claim 1 further comprising tuning the data transfermaximum associated with one or more paths, including: modifying, for oneor more of the plurality of paths, the data transfer maximum associatedwith the path; determining path performance for a period of time afterthe data transfer maximum associated with the path was modified; andresponsive to determining that path performance does not meet athreshold, reverting the data transfer maximum associated with the pathto its previous state.
 5. The method of claim 4 further comprising,responsive to determining that path performance does meet the thresholdafter the data transfer maximum associated with the path was modified,modifying the data transfer maximum associated with the path.
 6. Themethod of claim 1 wherein each path is utilized to service I/Ooperations of a particular type between the host and the target storagedevice.
 7. An apparatus for dynamic path selection in a storage network,the apparatus comprising a computer processor, a computer memoryoperatively coupled to the computer processor, the computer memoryhaving disposed within it computer program instructions that, whenexecuted by the computer processor, cause the apparatus to carry out:receiving, by a host that is coupled to a plurality of storage devicesvia a storage network, an I/O operation to be serviced by a targetstorage device; determining, for each of a plurality of paths betweenthe host and the target storage device, a data transfer maximumassociated with the path, wherein the data transfer maximum specifies acumulative amount of data that may be associated with I/O operationspending on the path; determining, for one or more of the plurality ofpaths, a number of I/O operations pending on the path; and selecting atarget path for transmitting the I/O operation to the target storagedevice in dependence upon the number of I/O operations pending on thepath and the data transfer maximum associated with the path.
 8. Theapparatus of claim 7 wherein selecting the target path further comprisesselecting a target path in further dependence upon the amount of data tobe transferred when executing the I/O operation.
 9. The apparatus ofclaim 7 further comprising computer program instructions that, whenexecuted by the computer processor, cause the apparatus to carry out:determining, for each of the plurality of paths, whether a sum of theamount of data to be transferred when executing the I/O operation andthe cumulative amount of data to be transferred by I/O operationspending on the path is greater than the data transfer maximum associatedwith the path; and responsive to determining, for each of the pluralityof paths, that the sum of the amount of data to be transferred whenexecuting the I/O operation and the cumulative amount of data to betransferred by I/O operations pending on the path is greater than thedata transfer maximum associated with the path, delaying a selection ofthe target path.
 10. The apparatus of claim 7 further comprisingcomputer program instructions that, when executed by the computerprocessor, cause the apparatus to carry out tuning the data transfermaximum associated with one or more paths, including: modifying, for oneor more of the plurality of paths, the data transfer maximum associatedwith the path; determining path performance for a period of time afterthe data transfer maximum associated with the path was modified; andresponsive to determining that path performance does not meet athreshold, reverting the data transfer maximum associated with the pathto its previous state.
 11. The apparatus of claim 10 further comprisingcomputer program instructions that, when executed by the computerprocessor, cause the apparatus to carry out, responsive to determiningthat path performance does not meet the threshold after the datatransfer maximum associated with the path was modified, modifying thedata transfer maximum associated with the path.
 12. The apparatus ofclaim 7 wherein each path is utilized to service I/O operations of aparticular type between the host and the target storage device.
 13. Acomputer program product for dynamic path selection in a storagenetwork, the computer program product disposed upon a computer readablemedium, the computer program product comprising computer programinstructions that, when executed, cause a computer to carry out:receiving, by a host that is coupled to a plurality of storage devicesvia a storage network, an I/O operation to be serviced by a targetstorage device; determining, for each of a plurality of paths betweenthe host and the target storage device, a data transfer maximumassociated with the path, wherein the data transfer maximum specifies acumulative amount of data that may be associated with I/O operationspending on the path; determining, for one or more of the plurality ofpaths, a number of I/O operations pending on the path; and selecting atarget path for transmitting the I/O operation to the target storagedevice in dependence upon the number of I/O operations pending on thepath and the data transfer maximum associated with the path.
 14. Thecomputer program product of claim 13 wherein selecting the target pathfurther comprises selecting a target path in further dependence upon theamount of data to be transferred when executing the I/O operation. 15.The computer program product of claim 13 further comprising computerprogram instructions that, when executed, cause the computer to carryout: determining, for each of the plurality of paths, whether a sum ofthe amount of data to be transferred when executing the I/O operationand the cumulative amount of data to be transferred by I/O operationspending on the path is greater than the data transfer maximum associatedwith the path; and responsive to determining, for each of the pluralityof paths, that the sum of the amount of data to be transferred whenexecuting the I/O operation and the cumulative amount of data to betransferred by I/O operations pending on the path is greater than thedata transfer maximum associated with the path, delaying a selection ofthe target path.
 16. The computer program product of claim 13 furthercomprising computer program instructions that, when executed, cause thecomputer to carry out tuning the data transfer maximum associated withone or more paths, including: modifying, for one or more of theplurality of paths, the data transfer maximum associated with the path;determining path performance for a period of time after the datatransfer maximum associated with the path was modified; and responsiveto determining that path performance does not meet a threshold,reverting the data transfer maximum associated with the path to itsprevious state.
 17. The computer program product of claim 16 furthercomprising computer program instructions that, when executed, cause thecomputer to carry out, responsive to determining that does meet thethreshold after the data transfer maximum associated with the path wasmodified, modifying the data transfer maximum associated with the path.